This invention generally relates to a circuit and a method for protecting a horizontal synchronous signal in a PCM signal decoding apparatus for decoding or demodulating a PCM signal formed as a standard television signal.
A PCM (pulse code modulation) system has been adopted in place of conventional analog recording/reproducing techniques for recording and reproducing various information on and from a magnetic recording tape. In a PCM system, the frequency range and the dynamic range are respectively dependent on the sampling period and the number of quantization bits. Therefore, it is possible to widen the frequency range and the dynamic range to the extent permitted by the range of the recording medium and the conversion system. In order to satisfactorily perform PCM recording and reproducing, it is necessary to use a recording/reproducing apparatus having a wide frequency range. A video tape recorder (referred to as VTR hereafter) is usually used as the apparatus having the necessary wide frequency range.
Since VTRs have been popular, it is now the general practice to record and reproduce audio signals as PCM signals having the same or a similar format as a standard TV (television) signal. Various standards for the signal format and processing method of such a PCM signal, established in June, 1979 by Electronic Industries Association of Japan (an incorporated body), are found in technical file STC-007 titled "Consumer use PCM encoder-decoder" issued by the Association.
In a helical scan type VTR, one of two rotary heads is selectively used by switching. As a result of switching, the horizontal synchronous signal (referred to as H. synch hereafter) reproduced via such rotary heads has a tendency to suffer from undesirable skew phenomena. Namely, the time interval between adjacent H. synch pulses has a tendency to be much shorter or longer than a predetermined desired interval. The maximum error due to skew is 20 microseconds or so. In order to prevent the reproducing system from malfunctioning, an H. synch supplementing circuit has been employed for automatically compensating for any dropped H. synch pulse. Although the conventional H. synch supplementing circuit satisfactorily functions when the time interval between adjacent H. synch pulses is shorter than the predetermined interval, it operates in an undesirable manner when the time interval is longer than the predetermined interval. If the time interval between adjacent H. synch pulses is longer than the predetermined interval, the supplementing circuit malfunctions, by continuously dropping out H. synch pulses.